Diabetes is characterized by impaired insulin secretion manifesting itself among other things by an elevated blood glucose level in the diabetic patient. Underlying defects lead to a classification of diabetes into two major groups: type I diabetes (or insulin dependent diabetes mellitus, IDDM), which arises when patients lack insulin-producing beta-cells in their pancreatic glands, and type 2 diabetes (or non-insulin dependent diabetes mellitus, NIDDM), which occurs in patients with an impaired beta-cell insulin secretion and/or alterations in insulin action.
Type 1 diabetic patients are currently treated with insulin, while the majority of type 2 diabetic patients can be treated with agents that stimulate beta-cell function or with agents that enhance the tissue sensitivity of the patients towards insulin. Over time, almost one-half of type 2 diabetic subjects lose their response to these agents and then must be placed on insulin therapy. The drugs presently used to treat type 2 diabetes include alpha-glucosidase inhibitors (PRECOSE®, (acarbose) voglibose and MIGLITOL®), insulin sensitizers (e.g., AVANDIA™ (rosiglitazone), ACTOS™ (pioglitazone) and REZULIN™ (troglitazone), insulin secretagogues (sulfonylureas (“SFUs”) and other agents that act by the ATP-dependent K+ channel), and GLUCOPHAGE™ (metformin HCl).
Alpha-glucosidase inhibitors. Alpha-glucosidase inhibitors reduce the excursion of postprandial glucose by delaying the absorption of glucose from the gut. These drugs are safe and provide treatment for mild to moderately affected diabetic subjects. However, gastrointestinal side effects have been reported in the literature and limit their effectiveness.
Insulin sensitizers. Insulin sensitizers are drugs that enhance the body's response to insulin. Thiozolidinediones such as AVANDIA™ (rosiglitazone) and ACTOS™ activate peroxisome proliferator-activated receptor (PPAR) gamma and modulate the activity of a set of genes that have not been well described. Hepatic effects (e.g., drug induced hepatotoxicity and elevated liver enzyme levels) do not appear to be a significant problem in patients using AVANDIA™ and ACTOS™. Even so, liver enzyme testing is recommended every two months in the first year of therapy and periodically thereafter. AVANDIA™ and ACTOS™ treatments are associated with fluid retention, edema, and weight gain. AVANDIA™ is not indicated for use with insulin because of concern about congestive heart failure. REZULIN™ (troglitazone), the first drug in this class, was withdrawn because of elevated liver enzyme levels and drug-induced hepatotoxicity.
Insulin secretagogues. Sulfonylureas (SFUs) and the non-sulfonylureas, Nateglinide and Pepaglinide act through the ATP-dependent potassium channel to cause glucose-independent insulin secretion. These drugs are standard therapy for type 2 diabetics that have mild to moderate fasting hyperglycemia. The insulin secretagogues have limitations that include a potential for inducing hypoglycemia, weight gain, and high primary and secondary failure rates. Ten to 20% of initially treated patients fail to show a significant treatment effect (primary failure). Secondary failure is demonstrated by an additional 20–30% loss of treatment effect after six months of treatment with insulin secretagogues. Insulin treatment is required in 50% of the insulin secretagogues responders after 5–7 years of therapy (Scheen et al., Diabetes Res. Clin. Pract. 6:533–543, 1989). Nateglinide and Pepaglinide are short-acting drugs that need to be taken three times a day. They are used only for the control of post-prandial glucose and not for control of fasting glucose.
GLUCOPHAGE™ is a biguanide that lowers blood glucose by decreasing hepatic glucose output and increasing peripheral glucose uptake and utilization. The drug is effective at lowering blood glucose in mildly and moderately affected subjects and does not have a side effect of weight gain or a potential to induce hypoglycemia. However, GLUCOPHAGE™ has a number of side effects, including gastrointestinal disturbances and lactic acidosis. GLUCOPHAGE™ is contraindicated in diabetics over the age of 70 and in subjects with impaired renal or liver function. Finally, GLUCOPHAGE™ has the same primary and secondary failure rates as the insulin secretagogues.
Insulin treatment is instituted after diet, exercise, and oral medications have failed to control blood glucose adequately. This treatment has several drawbacks: it is an injectable, it can produce hypoglycemia, and it can cause weight gain. The possibility of inducing hypoglycemia with insulin limits the extent that hypoglycemia can be controlled.
Problems with current treatments necessitate new therapies to treat type 2 diabetes. In particular, new treatments to retain normal (i.e., glucose-dependent) insulin secretion are needed. Given glucagon-like peptide-1's (“GLP-1”) role in promoting glucose-regulated insulin secretion in the pancreas, GLP-1 receptor agonists are potentially valuable in the treatment of such diseases. Moreover, glucagon receptor antagonists should prove valuable in treating type 2 diabetes given glucagon's role in elevating plasma glucose by stimulating hepatic glycogenolysis and gluconeogenesis.
GLP-1 and glucagon are members of a family of structurally related peptide hormones, the glucagon/secretin family. Within this family, GLP-1(7–36) and GLP-1(7–37) (30 amino acids and 31 amino acids, respectively) and glucagon (30 amino acids) constitute a highly homologous set of peptides. In addition, these two hormones originate from a common precursor, preproglucagon which, upon tissue-specific processing, leads to production of GLP-1 predominantly in the intestine and glucagon in the pancreas. The receptors for these two peptides are homologous (58% identity) and belong to the family of G-protein coupled receptors.
GLP-1 and glucagon both play major roles in overall glucose homeostasis. GLP-1 lowers plasma glucose concentrations mediated by glucose dependent insulin secretion, whereas glucagon increases plasma glucose concentrations. Given the important roles of both GLP-1 and glucagon in maintaining normal blood glucose concentrations, there has been considerable interest in the identification of GLP-1 receptor agonists and glucagon receptor antagonists. Clinical studies have demonstrated the ability of GLP-1 infusion to promote insulin secretion and to normalize plasma glucose in diabetic subjects. However, GLP-1 is rapidly degraded and has a very short half-life in the body. Furthermore, GLP-1 causes gut motility side effects at or near its therapeutic doses. Therefore, GLP-1 itself has significant limitations as a therapeutic agent, and modified versions of the peptide with enhanced stability are being pursued. Non-peptide agonists of the GLP-1 receptor have not been described to date.
Peptide analogs of glucagon have been identified which act as glucagon antagonists and reduce hyperglycemia in diabetic rats. However, no peptide glucagon antagonist has, moved beyond preclinical development. A number of structurally diverse non-peptide glucagon receptor antagonists have been reported in the scientific and patent literature. However, attempts to identify small molecule inhibitors of the glucagon receptor have met with limited success in vivo. The only antagonist of glucagon action known to be active in a clinical study is a compound identified as BAY 27-9955. A potential side effect of glucagon antagonism is hypoglycemia.
Because of the potential side effects associated with administering either a GLP-1 receptor agonist or a glucagon receptor antagonist alone, a combination therapy would have an advantage of maintaining the desired lowering of blood glucose while reducing the side effects. Co-administration, however, requires a single formulation and delivery approach that yields appropriate pharmokinetic profiles for both peptides. This could be a major obstacle to the development of such a therapeutic.
Based on the foregoing, considerable potential exists for a single therapeutic peptide functioning as both a GLP-1 agonist and a glucagon antagonist in vivo.